The authors agree that the surname derives from the personal name Marsilius, linked to the figure of the Saracen king who, according to the French epic tradition, fought against Roland in Spain and who also appears in Ariosto's Orlando furioso. It is therefore a name that reached Italy through the medieval epic texts of French origin.

E. De Stefani notes that the etymology admits two readings: it may be read as a demonym linked to the French city of Marseilles, or else as Marcilius, a diminutive of Marcus. He adds that people with this name are documented in Friuli from the 13th century, and that in time the name became a surname with a patronymic function.

A. Bongioanni, for his part, considers it unlikely that in the Middle Ages the name of an enemy of the Christian faith would be adopted as a given name, even a literary figure. He further notes that in Venice, San Marsilian corresponds to San Marziale, and that the etymology from Marseilles proposed by Fumagalli is not entirely certain.

As for the geographical distribution, De Stefani places the epicentre of the surname at Sutrio, with a sporadic presence in other Friulian communities, at Intissans —a hamlet of Verzegnis, in Carnia— and in Trieste. Bongioanni documents the forms Marsilio and Marsigio in Piedmont and in Friuli, and Marsilli in the Bologna area.